[extra-01] training has a period validity

We move from start/end date to a period date range to represent the
interval where a training take place.

Besides that, new constraints are added to ensure a trainings of a user
can't overlap. These constraints reflect in changeset validation.

priv/repo/migrations/20210627114504_create_gist_extension.exs

@@ -0,0 +1,11 @@
+defmodule Wabanex.Repo.Migrations.CreateGistExtension do
+  use Ecto.Migration
+
+  def up do
+    execute("CREATE EXTENSION IF NOT EXISTS \"btree_gist\";")
+  end
+
+  def down do
+    execute("DROP EXTENSION \"btree_gist\";")
+  end
+end